1. Field of the Invention
The invention relates generally to measurement devices and, in particular, to devices for measuring the dimension of an object such as the diameter of spherical components. Still more particularly, the invention relates to gauges for use in prosthetic applications such as the measurement of the spherical diameter of a femoral head and/or implantable femoral head components.
2. Description of the Prior Art
The use of measurement devices to determine the size of a patient's femoral head is essential in order to enable the use of appropriately sized prosthetic components. For example, during a total hip replacement the size of the patient's femoral head must be determined so that an appropriately sized prosthetic femoral head may be provided on the femoral stem component and also so that the appropriate acetabular cup size is utilized to mate with the implanted femoral head.
It is known in the prior art to use a linear caliper for the measurement of the outside diameter of spherical components such as femoral heads. The same or a similar caliper may be used to measure the inside diameter of the acetabulum. One example of such a caliper is the No. 3895 Townley Femur Caliper available from Zimmer, Inc. of Warsaw, Ind. While useful, this type of caliper can only measure a single dimension at a time and is highly sensitive to the correct placement of the caliper on the femoral head. It should be apparent that the measurement of a spherical dimension with a linear instrument is subject to inconsistent readings unless extreme care is taken in making the measurements. However, even with extreme care, numerous measurements must be made and the caliper must be adjusted for each measurement. Another disadvantage is that the user must often use both hands for proper operation of such calipers.
Other instruments for measuring the diameter of spherical prosthetic heads include open-faced templates and contour gauges provided in a range of discrete sizes so that the surgeon must select the gauge closest in size to the actual head being measured. These are also linear devices subject to the foregoing inaccuracies and also subject to the obvious difficulty of using the gauges when there is a slight mismatch between the available discrete sizes and the actual size of the head being measured. Examples of prior art contour gauges and femoral template sets are also available from Zimmer, Inc.
Some spherical head gauges are available as a set of discretely sized, closed circular openings which are used by matching the head being measured to the circular opening having the best fit. These types of gauges are analogous to the aforementioned contour gauges with the obvious difference that the contour gauge only provides an arcuate portion of a particular circular opening while the circular gauge provides the entire circular opening.
In view of the disadvantages associated with prior art spherical prosthetic gauges, it is an object of this invention to provide a prosthetic gauge for measuring the size of spherical prosthetic components with more facility than available with prior art components.
It is another object of this invention to provide a prosthetic gauge adapted for manual use and capable of providing accurate measurement of orthogonal diameters without being as sensitive to gauge placement on the spherical member being measured as prior art units.
It is a further object of this invention to provide a prosthetic gauge capable of single hand operation.